Antarctic Space Science - Publications

2004

A study of Pc-5 ULF oscillations

Authors: Hudson, M.; Denton, R.; Lessard, M.; Miftakhova, E.; Anderson, R.

Journal: Annales Geophysicae, vol. 22, Issue 1, pp.289-302

Date: Jan 2004

Abstract: . A study of Pc-5 magnetic pulsations using data from the Combined Release and Radiation Effects Satellite (CRRES) was carried out. Three-component dynamic magnetic field spectrograms have been used to survey ULF pulsation activity for the approximate fourteen month lifetime of CRRES. Two-hour panels of dynamic spectra were examined to find events which fall into two basic categories: 1) toroidal modes (fundamental and harmonic resonances) and 2) poloidal modes, which include compressional oscillations. The occurence rates were determined as a function of L value and local time. The main result is a comparable probability of occurence of toroidal mode oscillations on the dawn and dusk sides of the magnetosphere inside geosynchronous orbit, while poloidal mode oscillations occur predominantly along the dusk side, consistent with high azimuthal mode number excitation by ring current ions. Pc-5 pulsations following Storm Sudden Commencements (SSCs) were examined separately. The spatial distribution of modes for the SSC events was consistent with the statistical study for the lifetime of CRRES. The toroidal fundamental (and harmonic) resonances are the dominant mode seen on the dawn-side of the magnetosphere following SSCs. Power is mixed in all three components. In the 21 dusk side SSC events there were only a few examples of purely compressional (two) or radial (one) power in the CRRES study, a few more examples of purely toroidal modes (six), with all three components predominant in about half (ten) of the events.

Two-dimensional structure of long-period pulsations at polar latitudes in Antarctica

Authors: Yagova, N. V.; Pilipenko, V. A.; Lanzerotti, L. J.; Engebretson, M. J.; Rodger, A. S.; Lepidi, S.; Papitashvili, V. O.

Journal: Journal of Geophysical Research, Volume 109, Issue A3, CiteID A03222

Date: Mar 2004

Abstract: Two-dimensional (2-D) statistical distributions of spectral power and coherence of polar geomagnetic variations with quasi-periods about 10 min are analyzed using data from magnetometer arrays in Antarctica. Examination of the 2-D patterns of spectral power and coherence shows the occurrence of significant variations in geomagnetic power levels but with low spatial coherence near the cusp projection and in the auroral region. At the same time, low-amplitude pulsations, which we coin Picap3 pulsations, are very coherent throughout the polar cap. The region occupied by coherent Picap3 pulsations is shifted toward local MLT night from the geomagnetic pole and is decoupled from the regions of auroral and cusp ULF activity. The spectral power varies with time at polar latitudes in a manner different from that at auroral latitudes. Diurnal variations of power at different stations at the same geomagnetic latitude exhibit different behavior depending on the station's position relative to geomagnetic and geographic poles. This asymmetry is shown to be partly attributed to the variations of the ionospheric conductance. The primary source of polar pulsations is probably related to intermittent magnetosheath turbulence and tail lobe oscillations, though a particular propagation mechanism has not as yet been identified.

Latitudinal and seasonal variations of quasiperiodic and periodic VLF emissions in the outer magnetosphere

Authors: Engebretson, M. J.; Posch, J. L.; Halford, A. J.; Shelburne, G. A.; Smith, A. J.; Spasojević, M.; Inan, U. S.; Arnoldy, R. L.

Journal: Journal of Geophysical Research, Volume 109, Issue A5, CiteID A05216

Date: May 2004

Abstract: We have analyzed ELF-VLF receiver and search coil magnetometer data from five Antarctic stations from 1998 and 1999 to study quasiperiodic emissions (QPs) and periodic emissions (PEs), which occur as ULF-range modulations of ELF-VLF signals between 0.5 kHz and ~4 kHz. QPs are modulated at frequencies of ~20-50 mHz, and PEs are modulated at frequencies of ~200-500 mHz. The stations used covered a range of magnetic latitudes from -62° (Halley) to -74° (South Pole Station); three automated geophysical observatories (AGOs) were located at intermediate latitudes. Consistent with earlier studies, most QPs were observed with magnetic pulsations of identical period in the Pc3 range (type I QPs). Of those QPs not observed with simultaneous magnetic pulsations (type II QPs), nearly all were accompanied by PEs. Type I QPs, PEs, and events during which both appeared together (QPPEs) were found to have different latitudinal, seasonal, and diurnal occurrence patterns: QPs of both types were more likely to occur between -65° and -70° magnetic latitude, while PEs occurred more often around -60° magnetic latitude. QPs were more common during the months of October though March, while PEs were more common during the months of May through September. QPs, whether with or without simultaneous PEs or magnetic pulsations, were predominantly a dayside phenomenon, with a broad maximum near local noon. The occurrence of QPs unaccompanied by PEs was restricted to the dayside, however, while a small number of QPPEs appeared even during nighttime hours. PEs, on the other hand, could be seen at all local times, but with latitudinally dependent diurnal patterns. Most higher-latitude QPs were type I events (observed with magnetic pulsations), while type II QP events (without simultaneous magnetic pulsations) occurred relatively more often at lower latitudes. A case study from 1 August 1999 using wideband data from South Pole and Halley provides evidence of a transition from echoing whistler activity to PE activity and then to QP activity and suggests a causal relationship.

Energy budget of Alfven wave interactions with the auroral acceleration region

Authors: Pilipenko, V.; Fedorov, E.; Engebretson, M. J.; Yumoto, K.

Journal: Journal of Geophysical Research, Volume 109, Issue A10, CiteID A10204

Date: Oct 2004

Abstract: We consider the interaction of Alfven waves with the auroral acceleration region (AAR), which is characterized by a mirror resistance and a related electric potential drop that supports a field-aligned upward current and the acceleration of electrons. An Alfven wave incident on the AAR from the magnetosphere partially reflects back and partially penetrates into the AAR. An analytical treatment of the interaction of Alfven waves with the combined magnetosphere-AAR-topside ionosphere-E layer system immersed into a converging dipole-like magnetic field has been made within the ``thin'' AAR approximation. The rate of wave reflection/transmission is estimated to be critically dependent on the wave transverse scale. Incident Alfven waves with spatial scales about the Alfven damping scale λA are absorbed most effectively in the AAR. Magnetospheric Alfven waves penetrating into the AAR can produce oscillatory variations of the field-aligned potential drop and field-aligned electron acceleration. Modeling the spatial spectrum of an Alfven burst by a power-law dependence $\propto$ k-p in the interval between low and high cutoff wave numbers indicates that the estimated rate of total wave power absorbed by the AAR is significant, up to 30-50% depending on p, when the ionospheric projection of the low cutoff wave number is on the order of the scale λA.

Interplanetary magnetic field control of dayside transient event occurrence and motion in the ionosphere and magnetosphere

Authors: Korotova, G.; Sibeck, D.; Singer, H.; Rosenberg, T.; Engebretson, M.

Journal: Annales Geophysicae, vol. 22, Issue 12, pp.4197-4202

Date: Dec 2004

Abstract: The pressure pulse model for dayside transient ionospheric events predicts dawnward moving events at and prior to local noon during periods of spiral interplanetary magnetic field (IMF) orientation, but duskward moving events at and after local noon during rarer periods of orthospiral IMF orientation. We use this model to interpret ground and geosynchronous magnetometer observations of a duskward-moving transient event that occurred on 10 August 1995 during a period of orthospiral IMF orientation. We then survey geosynchronous GOES-8, 9, and 10 magnetometer observations to determine the directions of motion for 67 isolated magnetic impulse events seen in South Pole magnetograms from 1995-1999. The occurrence patterns and directions of motion inferred from both case and statistical studies are consistent with pressure pulse model predictions.

Global geomagnetic response to a sharp compression of the magnetosphere and IMF variations on October 29, 2003

Authors: Solovyev, S. I.; Moiseyev, A. V.; Mullayarov, V. A.; Du, A.; Engebretson, M.; Newitt, L.

Journal: Cosmic Research, Volume 42, Issue 6, pp.597-606

Date: Nov 2004

Abstract: We study the response of the ionosphere and magnetosphere to a sudden commencement (SC) on October 29, 2003, at 06:11 UT. It is shown that the geomagnetic response had the form of two successive stages. In the first ˜5 min after the SC, a strong intensification of a two-vortex current system of the DP2 type was observed in latitudes ˜67°-65°, with variations of Δ H -4000 nT (+700 nT). At the same time, energetic electrons were injected without dispersion to geosynchronous orbits simultaneously in the sectors ˜16, ˜04, and ˜07 MLT. In the subsequent 5 15 min, a new intensification of the western electrojet took place in all time sectors at latitudes ˜70°. Around midnight, this electrojet was extended in the poleward direction up to the polar cap latitudes (Φ’ ≈ 75°-83°). It had an unusually high velocity of extension (up to ˜5.0 km/s) and was accompanied by typical dispersionless substorm injections, but only at meridians ˜04 and 07 MLT. From comparing the development of electrojets with the data of satellite observations in the solar wind and magnetosphere, we suggest that ˜3 5 min after the SC onset a dipolization of the magnetic field at the geosynchronous orbit occurred. It was connected with the decay of the current flowing across the magnetotail. The subsequent extension of the region of current decay into the tail up to 150 R E proceeded with a velocity of ≥1000 km/s, which exceeds the known velocities of such an extension by a factor of ˜5.

Alfven wave modulation of the auroral acceleration region

Authors: Fedorov, E. N.; Pilipenko, V. A.; Engebretson, M. J.; Rosenberg, T. J.

Journal: Earth, Planets and Space, Volume 56, p. 649-661.

Date: Jul 2004

Abstract: We consider the interaction of Alfven waves with the auroral acceleration region (AAR). The AAR is character-ized by an electric potential drop that supports a field-aligned upward current and the acceleration of precipitating electrons. An Alfven wave incident on the AAR from the magnetosphere partially reflects back and partially pen-etrates into the AAR. The rate of wave reflection/transmission is estimated to be critically dependent on the wave transverse scale. Magnetospheric Alfven waves penetrating into the AAR can produce oscillatory variations of the field-aligned potential drop, thus constituting a new mechanism of ULF modulation of electron acceleration. Estimates of the potential drop modulation by Alfven waves are provided within the "thin" AAR approximation, which is valid for a wide range of wave and plasma parameters. The proposed mechanism will produce nearly simulta-neous ULF magnetic and riometric variations at auroral latitudes. Occurrence of the AAR-associated resonator in the auroral topside ionosphere between the bottom boundary of the AAR and the E-layer may cause oscillatory frequency dependence of electron acceleration modulations in the range around fractions of a Hz. Another feature of the mechanism considered is the critical dependence of the ratio between the magnetic and riometric signals on the transverse scale of the disturbance. The predicted effects are to be searched for in the simultaneous data of IRIS multi-beam riometers and magnetometers.